The present invention relates to a braking-pressure control system, especially for a road vehicle.
British Published Patent Application No. 2 281 364 concerns a braking-pressure control system for a hydraulic dual-circuit braking system having a pneumatic power-brake unit (brake booster). A valve configuration and a return pump are provided for each brake circuit. In addition, an electronic control unit is provided, which generates signals for controlling switch-over valves (pilot valves), precharging control valves, intake and exhaust valves, as well as the return pumps; the signals are from a processing (processor) of sensor signals, which include the information on the manner of brake-pedal operation. A pneumatic brake booster includes two chambers separable from each other via a valve, one of which is operated as a low-pressure chamber and the other as a working or driving chamber and a saturation valve, with which the working chamber of the brake booster can be aerated. The apparent object of the braking-pressure control system is to ensure the highest possible values for vehicle deceleration, even in response to an automatically-controlled, full application of the brakes. In the meantime, such automatically-controlled full braking has become known under the name of xe2x80x9cbraking assistantxe2x80x9d. The braking pressure necessary for automatically applying the brakes fully is generated with the aid of the valve configuration and the return pumps, and therefore, by suitably controlling the unit subsequently designated as a hydraulic unit (hydraulic modulator).
British Patent Application No. 2 281 364 A further describes spot braking, i.e. braking during normal traffic events controlled by the driver, without the support of the hydraulic modulator (cf. page 12, lines 34-37). However, the braking force is therefore boosted only by the pneumatic brake booster in this case, which may occur in normal traffic events. This pneumatic brake booster must be correspondingly designed to reach the maximum required braking force. A disadvantage of such a pneumatic brake booster is its size, which is determined by braking pressures that must be reached. Another disadvantage is that the solenoid valves of the valve configuration, especially the pilot and the precharging control valves, must be designed for the comparably high main-cylinder pressures, which can be generated using such a pneumatic brake booster. In certain brake boosters, these pressures may be on the order of up to 250 bar.
The device for controlling an ABS-TCS system, by which it is possible to replace the vacuum brake booster completely or partially with hydraulic brake boosting. The layout of the braking system may be considered to be analogous to that of British Patent No. 2 281 364. The braking force is hydraulically boosted by selectively controlling the valve configuration and the return pumps. However, the result of completely replacing the pneumatic brake booster is, that the return pumps of the braking system must be started up in response to each braking operation, in which an increased or boosted braking pressure must be generated. This has the disadvantage of the return operation leading to pedal pulsations, which are especially annoying at low pedal forces. In addition, noises originating from operating the pump decrease the ride comfort. Although comfort may only play a subordinate role in an, emergency situation, a loss of comfort is hardly acceptable in normal vehicle operation. Also, a braking system based exclusively on hydraulic brake boosting offers less redundancy in the case of a breakdown or an error in the hydraulic modulator.
It is believed that pneumatic brake boosters used up to this point are not as well suited for an optimal design of the combined pneumatic and hydraulic braking system.
An object of an exemplary embodiment of the present invention involves combining the performance of the braking system discussed above, cylinder, with the opposing requirements for comfort (no pedal pulsations, as little disturbing noise as possible) and for taking up as little space as possible. An object of an exemplary embodiment of the present invention is to provide a braking-pressure control system that is optimized in view of these opposing requirements.
An advantage regarding the design of the braking-pressure control system is that only the pneumatic brake booster is operated in a range, in which only comparatively moderate braking pressures are needed. Upon reaching its saturation point, i.e. when the braking-pressure support cannot be further increased using the pneumatic brake booster, an additional boost in the braking pressure is generated with the aid of the hydraulic modulator. In this case, a fundamental difference from the function of the xe2x80x9cbraking assistantxe2x80x9d is that a proportional dependence between the control pressures and the braking pressures in the wheel-brake cylinders exists (the braking pressures in the wheel-brake cylinders are largely proportional to the control pressures) over a wide operating range of the braking system, that is, for all possible brake-pedal positions, and therefore, for all possible control pressures selected by the driver of the vehicle. It is believed that this desired dependence is necessary for the driver to be able to effectively regulate the braking force quantitatively (proportion the braking force).
In this context, one must consider that the desire for proportionality of the braking force requires a rather flat characteristic curve, whereas a large boost in the braking force requires a steep characteristic curve. In this regard, different, opposing requirements must therefore also be considered in the design of the braking-pressure control system.
A particular difficulty in combining a pneumatic brake booster with a hydraulic brake booster for braking operations during normal driving events, i.e., spot braking, is to attain a defined (discrete) transition between the pneumatic and the hydraulic brake booster, which is as unnoticeable as possible to the vehicle driver. At the same time, the characteristic curve of the entire braking system should satisfy all of the existing comfort and safety requirements over all required braking pressures. Accordingly, another aspect of an exemplary embodiment of the present invention provides a braking-pressure control system of the type mentioned with an arrangement or structure for determining and evaluating the attainment of the saturation point of the pneumatic brake booster.
An advantage of the braking-pressure control system according to an exemplary embodiment of the present invention is that the pneumatic brake booster requires less space than other pneumatic, tandem, vacuum brake boosters, at least when they provide a boost similar to that of the presently described braking-pressure control system. At the same time, combining the pneumatic brake booster with a hydraulic brake booster allows even higher brake pressures to be generated in the wheel-brake cylinders. In comparison with smaller vacuum boosters, the pneumatic brake booster according to an exemplary embodiment of the present invention provides a higher boost, because its characteristic curve is designed to be steeper in the operating range, up to the saturation point.
An additional advantage of an exemplary embodiment of the present invention is that the braking system as a whole remains fully functional in response to both a vacuum breakdown and fading, since the hydraulic brake boosting can provide the entire braking-force boost in this case. The accompanying losses in comfort are acceptable, since this is certainly an exceptional case. In the same manner, the reduced performance of the pneumatic brake booster, e.g. in response to falling atmospheric pressure (driving in the mountains at an altitude of 3000 m), can be compensated for by the hydraulic brake boosting, This is also done only as needed and, as a rule, is not believed to be necessary in daily operation.
A further advantage of an exemplary embodiment of the present invention is that only comparatively low pressures can be generated in the main cylinder of the braking system (up to a maximum of 150 bar, in comparison with up to 250 bar previously), because of the small size of the pneumatic brake booster of an exemplary embodiment of the present invention. Accordingly, the solenoid valves of the hydraulic modulator must also only be designed for these correspondingly lower pressures. This reduces the outlay in designing and manufacturing the valves.
In comparison with a purely hydraulic braking system, the braking-pressure control system according to an exemplary embodiment of the present invention offers a higher level of comfort, since the return pumps must only be started up to generate braking pressure in the case of hard braking. In the typical braking-pressure range, the performance of the braking-pressure control system of an exemplary embodiment of the present invention corresponds to that of a system only having a vacuum booster, and therefore achieves a high level of pedal comfort, and can be quantitatively regulated in an effective manner. This allows the hydraulic modulator to be manufactured with reduced outlay, and therefore more cost-effectively, since its effect on the pedal comfort is no longer in the forefront.
Furthermore, further features of an exemplary embodiment of the present invention ensure that the braking force can be hydraulically supported in a selective manner. In comparison with using a conventional, small vacuum booster, the braking-pressure control system of an exemplary embodiment of the present invention also has the advantage of already providing a large braking-force boost at low braking pressures needed for spot braking.
FIG. 1 shows a schematic circuit diagram of an exemplary braking system.
FIG. 2 shows a control arrangement or unit.
FIG. 3a shows a pneumatic brake booster having an arrangement for sensing the saturation point.
FIG. 3b shows the pneumatic brake booster having another arrangement for sensing the saturation point.
FIG. 4 shows characteristic curves of various pneumatic brake boosters.
FIG. 5 shows an exemplary characteristic curve of a braking-pressure control system according to an exemplary embodiment of the present invention.
FIG. 6 shows an arrangement for detecting an error condition.